Process for preparing polytetrahydrofuran and derivatives thereof

ABSTRACT

Polytetrahydrofuran, copolymers of tetrahydrofuran and 2-butyne-1,4-diol, diesters of these polymers with C 2  -C 20  -monocarboxylic acids or monoesters of these polymers with C 1  -C 10  -monocarboxylic acids are prepared by polymerization of tetrahydrofuran in the presence of one of the telogens water, 1,4-butanediol, 2-butyne-1,4-diol, polytetrahydrofuran having a molecular weight of from 200 to 700 Dalton, a C 1  -C 10  -monocarboxylic acid or an anhydride of a C 2  -C 20  -monocarboxylic acid or a mixture of these telogens over a heterogeneous supported catalyst which comprises a catalytically active amount of an oxygen-containing molybdenum and/or tungsten compound on an oxidic support material and which has been calcined at from 5OO° C. to 1OOO° C. after application of the precursor compounds of the oxygen-containing molybdenum and/or tungsten compounds onto the support material precursor, wherein the catalyst is activated by treatment with a reducing agent before it is used as a polymerization catalyst.

This application is a 371 of PCT/EP97/05204 filed Sep. 23, 1997.

FIELD OF THE INVENTION

The present invention relates to an improved process for preparingpolytetrahydrofuran, copolymers of tetrahydrofuran and2-butyne-1,4-diol, diesters of these polymers with C₂ -C₂₀-monocarboxylic acids or monoesters of these polymers with C₁ -C₁₀-monocarboxylic acids by polymerization of tetrahydrofuran in thepresence of one of the telogens water, 1,4-butanediol,2-butyne-1,4-diol, polytetrahydrofuran having a molecular weight of from200 to 700 Dalton, a C₁ -C₁₀ -monocarboxylic acid or an anhydride of aC₂ -C₂₀ -monocarboxylic acid or a mixture of these telogens over aheterogeneous supported catalyst which comprises a catalytically activeamount of an oxygen-containing molybdenum and/or tungsten compound on anoxidic support material and which has been calcined at from 500° C. to1000° C. after application of the precursor compounds of theoxygen-containing molybdenum and/or tungsten compounds onto the supportmaterial precursor.

Polytetrahydrofuran ("PTHF"), also known as poly(oxybutylene glycol), isa versatile intermediate in the plastics and synthetic fibers industriesand serves, inter alia, for the preparation of polyurethane, polyesterand polyamide elastomers for whose preparation it is used as diolcomponent. In addition, polytetrahydrofuran as well as some of itsderivatives is a valuable auxiliary in many applications, for example asdispersant or for deinking waste paper.

In industry, PTHF is advantageously prepared by polymerization oftetrahydrofuran over suitable catalysts in the presence of reagentswhose addition makes it possible to control the length of the polymerchains and thus to set the mean molecular weight to the desired value(chain-termination reagents or "telogens"). The control is effected hereby selection of type and amount of the telogen. Selection of suitabletelogens makes it possible to introduce additional functional groups atone end or both ends of the polymer chain. Thus, for example, themonoesters or diesters of PTHF can be prepared by using carboxylic acidsor carboxylic anhydrides as telogens. Other telogens, for example thosehaving two hydroxy groups such as 1,4-butanediol, 2-butyne-1,4-diol orlow molecular weight PTHF not only act as chain-termination reagents,but are also incorporated into the growing polymer chain of the PTHF. Inthis way, the PTHP can also be chemically modified. An example of thisis the use of the telogen 2-butyne-1,4-diol whose addition leads to thepresence of a proportion of C.tbd.C triple bonds in the polymer chainsof the PTHF.

PTHF modified in this way can be further altered chemically at thesepoints by means of the reactivity of these triple bonds, for example byhydrogenation of the triple bonds to double bonds, by subsequentgrafting-on of other monomers for adjusting the properties of thepolymer, crosslinking to form polymers having a comparatively rigidstructure, or other conventional procedures of polymer chemistry. Thecomplete hydrogenation of the triple bonds present is likewise possibleand generally leads to PTHF having a particularly low color number.

DE-A 44 33 606 describes a process for preparing PTHF, PTHF diesters ofC₂ -C₂₀ -monocarboxylic acids or PTHF monoesters of C₁ -C₁₀-monocarboxylic acids by polymerization of tetrahydrofuran over aheterogeneous catalyst in the presence of one of the telogens water,1,4-butanediol, PTHF having a molecular weight of from 200 to 700Dalton, a C₁ -C₁₀ -monocarboxylic acid or an anhydride of a C₂ -C₂₀-monocarboxylic acid or a mixture of these telogens, with the catalystbeing a supported catalyst which comprises a catalytically active amountof an oxygen-containing tungsten or molybdenum compound or a mixturethereof on an oxidic support material and which has been calcined atfrom 500° C. to 1000° C. after application of the precursor compounds ofthe oxygen-containing molybdenum and/or tungsten compounds onto thesupport material precursor.

WO 96/09335 teaches a process for preparing PTHF or PTHF monoesters ofC₁ -C₁₀ -monocarboxylic acids by polymerization of tetrahydrofuran overa heterogeneous catalyst in the presence of one of the telogens water,1,4-butanediol, PTHF having a molecular weight of from 200 to 700Dalton, a C₁ -C₁₀ -monocarboxylic acid or a mixture of these telogens,with the catalyst being a supported catalyst which comprises acatalytically active amount of an oxygen-containing tungsten ormolybdenum compound or a mixture thereof on an oxidic support materialand which has been calcined at from 500° C. to 1000° C. afterapplication of the precursor compounds of the oxygen-containingmolybdenum and/or tungsten compounds onto the support materialprecursor.

SUMMARY OF THE INVENTION

It is an object of the present invention to increase the catalyticactivity of the THF polymerization catalysts described in DE 44 33 606or WO 96/09335 in order to achieve higher polymer yields and/orspace-time yields, thus improving the economics of the process whichdepend decisively on the productivity of the catalyst.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

We have found that this object is achieved by an improved process forpreparing polytetrahydrofuran, copolymers of tetrahydrofuran and2-butyne-1,4-diol, diesters of these polymers with C₂ -C₂₀-monocarboxylic acids or monoesters of these polymers with C₁ -C₁₀-monocarboxylic acids by polymerization of tetrahydrofuran in thepresence of one of the telogens water, 1,4-butanediol,2-butyne-1,4-diol, polytetrahydrofuran having a molecular weight of from200 to 700 Dalton, a C₁ -C₁₀ -monocarboxylic acid or an anhydride of aC₂ -C₂₀ -monocarboxylic acid or a mixture of these telogens over aheterogeneous supported catalyst which comprises a catalytically activeamount of an oxygen-containing molybdenum and/or tungsten compound on anoxidic support material and which has been calcined at from 500° C. to1000° C. after application of the precursor compounds of theoxygen-containing molybdenum and/or tungsten compounds onto the supportmaterial precursor, wherein the catalyst is activated by treatment witha reducing agent before it is used as a polymerization catalyst.

Polymerization catalysts used in the process of the present inventionare supported catalysts which comprise an oxidic support material andoxygen-containing molybdenum or tungsten compounds or mixtures thereofas catalytically active compounds and which can, if desired,additionally be doped with sulfate and/or phosphate groups. To convertthem into their catalytically active form, the supported catalysts aresubjected to calcination at from 500° C. to 1000° C. after applicationof the precursor compounds of the catalytically active,oxygen-containing molybdenum and/or tungsten compounds onto the supportmaterial, with the support material and the precursor compound beingconverted into the catalyst.

The catalysts, their preparation and their use in the processes forpreparing polytetrahydrofuran, polytetrahydrofuran diesters of C₂ -C₂₀-monocarboxylic acids or polytetrahydrofuran monoesters of C₁ -C₁₀-monocarboxylic acids are described in detail in DE 44 33 606 or WO96/09335, which are hereby expressly incorporated by reference. The useof the telogen 2-butyne-1,4-diol is described in detail in the GermanPatent Application No. 19507399.1 (=PCT Application PCT/EP96/00702) andNo. 19527532.2 (=PCT Application PCT/EP96/03297).

According to the present invention, the activity of the catalysts isincreased by treatment with a reducing agent after calcination andbefore use as polymerization catalysts. The reducing agent is oxidizedat least partially to an oxidation product or a mixture of oxidationproducts.

Suitable reducing agents are in principle all reducing agents which donot leave any residues on the catalysts treated therewith or only leaveresidues which are inert in the polymerization of tetrahydrofuran andwhich do not adversely affect the use of the polymerization products.

In general, it is advantageous to use reducing agents which, during thereduction of the catalyst, are converted only into oxidation productswhich are inert toward the catalyst and escape in gaseous form. However,it is equally advantageous to use reducing agents whose oxidationproducts are not gaseous but can be removed from the catalyst with asolvent or suspension medium used in the reduction or with the liquidreducing agent and are inert toward the catalyst. It is in principlealso possible to use reducing agents whose oxidation products cannot beseparated from the catalyst as long as the oxidation products are inerttoward the catalyst and the components of the polymerization reaction.This procedure can even be advantageous, particularly when the oxidationproducts act as promoter for the catalyst, i.e. promote thepolymerization reaction, or when the advantage of one less process step,namely the removal of the oxidation products, outweighs the possibledisadvantage of a certain proportion of inert volume in the reactor.

Reducing agents which can be used are, for example, organic compoundswhich have a reducing action on the catalysts in their calcined form,for instance alcohols, aldehydes, carboxylic acids or carbohydrates,with bifunctional or polyfunctional compounds such as hydroxyacids,hydroxyaldehydes, polyalcohols, dialdehydes or polyaldehydes or diacidsor polyacids or salts of organic reducing agents, preferably theammonium salts, likewise being able to be used.

Examples of organic reducing agents which can be used according to thepresent invention are straight-chain or branched aldehydes having fromone to ten carbon atoms, for example formaldehyde or acetaldehyde,propionaldehyde, butyraldehyde, isobutyraldehyde or glyoxal,straight-chain, branched or cyclic monocarboxylic or dicarboxylic acidsor their salts, for example formic acid, ammonium formate, acetic acid,propionic acid, butyric acid, isobutyric acid, oxalic acid, lactic acid,benzoic acid, citric acid or ascorbic acid, straight-chain, branched orcyclic alcohols or carbohydrates such as methanol, ethanol, propanol,isopropanol or aldoses such as glucose.

Examples of inorganic reducing agents which can be used according to thepresent invention are hydrogen-containing compounds such as hydrides,for example alkali metal tetrahydridoborates such as sodiumtetrahydridoborate, 9-borabicyclononane, catecholborane, a solution ofdiborane in tetrahydrofuran or in other ethers, alkali metaltetrahydridoaluminates such as lithium tetrahydridoaluminate, or simplebinary hydrides such as the hydrides of the alkali or alkaline earthmetals, eg. lithium hydride, sodium hydride or calcium hydride.

However, it is equally possible to use compounds which do not containhydridic hydrogen but have a reducing action. Examples of such reducingagents are phosphites or hypophosphites such as ammonium phosphite orammonium hypophosphite or sulfites or hydrogensulfites such as ammoniumsulfite or ammonium hydrogensulfite. In this case, phosphate- orsulfate-doped catalysts are obtained automatically in the reducingtreatment. However, this procedure is not absolutely necessary in orderto prepare sulfate- or phosphate-doped catalysts; these can also beprepared by reducing treatment of a sulfate- or phosphate-dopedcatalyst. In general, the optimum amount of phosphate- or sulfate-dopantin the catalyst will not be exactly the amount obtained when using theamount of phosphites, hypophosphites, sulfites or hydrogensulfites whichis optimum for the reaction. In these cases, either the amount of thesedopants on the catalyst has to be increased to the desired value byaddition of sulfate or phosphate, or the reduction of the catalyst hasto be continued to the desired extent by further hydrogenation usinganother reducing agent.

Preferred reducing agents are hydrogen-containing gases such as purehydrogen or hydrogen in admixture with other gases. Particularpreference is given to using hydrogen either in pure form or dilutedwith an inert gas such as nitrogen or argon. For example, mixtures ofnitrogen and hydrogen are very suitable. Such mixtures can comprise upto about 60% by volume of hydrogen in nitrogen; hydrogen contents of upto 40% by volume or 20% by volume are likewise suitable. However, ahydrogen content of up to 10% by volume is generally sufficient. Thehydrogen content can also be increased gradually over the course of thereduction reaction in order to avoid an excessively strong exothermicreaction at the beginning. Thus, for example, an inert gas/hydrogenvolume ratio of about 99:1 (or even higher, for instance 99.5:0.5) canbe used at the beginning, and this is generally decreased over thecourse of the reduction since otherwise the reduction times requiredincrease. Ratios of, for example, 98:2, 95:5, 90:10, 80:20, 60:40, 50:50or even lower values right through to pure hydrogen can be setsuccessively or directly; a more finely stepped transition between themixtures having different hydrogen concentrations right through to acontinuous rise in the proportion of hydrogen can also be employed. Therate at which the proportion of hydrogen is increased and the finalvalue of this hydrogen content are advantageously set as a function ofthe heat liberated during the reduction so that excessive evolution ofheat is avoided. Evolution of heat is excessive, for example, when theheat of reaction liberated can no longer be removed by the coolingsystem of the reduction reactor. Heat evolution is also excessive, forexample, when the catalyst, as a result of the heat of reactionliberated, reaches temperatures which are detrimental to its propertiesin the polymerization, for instance when the catalyst melts, sinters orchanges thermally to at least some extent in some other way, for exampleby thermal decomposition or vaporization of organic constituents such asextrusion or tableting aids.

The treatment of the calcined catalyst with the reducing agent isgenerally carried out at from 20° C. to 500° C. If the reduction iscarried out using hydrogen-containing gases, the preferred temperatureis in the range from 100° C. to 400° C. However, if the reduction iscarried out using solid, liquid or dissolved reducing agents, thepreferred reduction temperature is in the range from 20 to 200° C.

If the reduction is not carried out by treating the catalyst withgaseous materials, the pressure used is generally unimportant. However,if in this case the reducing agent is converted completely or partiallyinto gaseous oxidation products during the reduction of the catalyst,the reaction pressure used should not hinder the formation of thesegaseous oxidation products: a pressure which is not overly high is thusgenerally advantageous. It can be, for example, from 0.1 to 5 bar(absolute). The reduction is preferably carried out under atmosphericpressure. If the reduction is carried out using gaseous reducing agents,it can be carried out at atmospheric pressure or increased pressure, forexample from 1 to 300 bar (absolute) and preferably from 1 to 50 bar(absolute).

The reduction time is generally from 10 minutes to 100 hours, preferablyfrom 30 minutes to 50 hours and particularly preferably from 1 hour to24 hours.

The reducing agent is generally used in an amount of from 0.01 to 100mol, in particular from 0.1 to 50 mol, per gram of the calcined catalystand preferably in an amount of from 0.1 to 10 mol per gram of thecalcined catalyst.

The reduction temperature, the reduction time and the amount of reducingagent which are optimum for a given composition of the calcined catalysthave to be determined empirically for the individual case within thegeneral ranges indicated by routine reduction experiments and reactiontests, since they depend on the amount of active component in thecatalyst, the type of active component, the type of support and the typeand amount of dopants. In general, the reduction is complete when theevolution of heat which usually occurs at the beginning of the reductionhas essentially abated, and this can be followed by a further reductiontime of from 5 minutes to 5 hours, advantageously from 10 minutes to 2hours.

The reduction can be carried out by treatment of the catalyst with aliquid containing the reducing agent. For example, the catalyst can betreated with a solution or suspension of the reducing agent in asuitable solvent or suspension medium. The selection of this solvent issubject to the general conditions that the catalyst has to be virtuallyinsoluble therein and that the reducing agent has to be sufficientlysoluble therein. The selection of the suspension medium is subject tothe general conditions that the catalyst has to be virtually insolubletherein and that the suspension medium has to be virtually inert towardthe reducing agent. There is no sharp dividing line between solution andsuspension since it is possible for part of the reducing agent to bedissolved and a further part to be suspended in undissolved form. Thesolvent or suspension medium can be selected routinely by means of asolubility table. Examples of solvents or suspension media are water oralcohols having from one to ten carbon atoms, eg. methanol, ethanol,propanol, isopropanol and butanol, aliphatic, branched or unbranched orcyclic ethers such as diethyl ether, di-n-butyl ether, methyl tert-butylether, ethyl tert-butyl ether, monoethylene, diethylene and triethyleneglycol dimethyl ethers or tetrahydrofuran or aliphatic, branched orunbranched or cyclic or aromatic hydrocarbons having from five to tencarbon atoms, eg. pentane, hexane, benzene, toluene, xylene, tetralin ordecalin.

The reduction can likewise be carried out by treating the catalyst witha liquid reducing agent without addition of a solvent.

For this reductive treatment of the catalyst with a reducing agentpresent in a liquid phase, the catalyst can be arranged as a fixed bedin a reactor and have the liquid passed through it or can be present asa suspension. The latter method can be advantageous, for example, if thecatalyst is in the form of powder and is only made into shaped bodiesafter the reduction step. Suitable reactors are, for example, stirredvessels or loop reactors having internal or external reflux. Thereduction is advantageously carried out directly in the polymerizationreactor.

The reduction can also be carried out by treating the solid catalystwith a solid reducing agent. For example, calcined catalyst powder canbe mixed with a solid reducing agent and shaped and then reduced byheating to the reduction temperature.

After the reduction of the catalyst with a reducing agent present in aliquid or solid phase, a washing step can be carried out in order toremove residues of the reducing agent or its oxidation products from thecatalyst. For this purpose, the catalyst can be slurried with a puresolvent which is able to dissolve the residues of the reductivetreatment which are to be removed from the catalyst, similar to theprocedure in the reduction, or can be washed with this solvent. Examplesof solvents which can be used are the abovementioned solvents for thereducing agents. It goes without saying that to remove oxidized reducingagent residues which are insoluble in the solvent used for this reducingagent, another solvent has to be used for the washing step. Theselection of this solvent is subject to the general conditions that thecatalyst has to be virtually insoluble therein and that the residues tobe removed have to be sufficiently soluble therein, and the solvent canbe selected routinely with the aid of a solubility table.

The reduction is preferably carried out by treating the catalyst with agaseous reducing agent and particularly preferably by treating thecatalyst with a mixture of hydrogen and nitrogen. For this purpose, thecatalyst in the form of powder or shaped bodies is advantageouslyarranged in a fixed-bed reactor and the hydrogen-containing gas mixtureis made to flow through the catalyst bed. The reactor has a temperatureregulation system which, on the one hand, is able to remove the heatliberated during the reaction and, on the other hand, can dissipate thereaction temperature required. After the hydrogenation, the catalyst canbe made into shaped bodies if this has not already been done before thehydrogenation.

EXAMPLES Preparation of the Catalysts

Catalyst A [20% WO₃ /TiO₂ ]

Catalyst A was prepared by adding 4400 g of titanium dioxide to asolution of 850 g of tungstic acid in 4860 g of 25% strength aqueous NH₃solution. This mixture was kneaded for 60 minutes and then dried for 12hours at 120° C. The powder obtained after sieving was tableted and theresulting pellets (7×7×3 mm) were subsequently calcined for 2 hours at650° C. The catalyst had a tungsten content, calculated as tungstentrioxide, of 20% by weight, based on the total weight of the catalyst.

Catalyst B

Catalyst B was prepared from catalyst A: 30 g of catalyst A were heatedat 300° C. in a quartz tube at atmospheric pressure in a stream of purehydrogen (35 l/h) for 30 hours and subsequently cooled to roomtemperature under nitrogen.

Catalyst C

Catalyst C was prepared from catalyst A: 50 g of catalyst A were heatedat 300° C. in a quartz tube at atmospheric pressure in a stream of purehydrogen (35 l/h) for 6 hours and subsequently cooled to roomtemperature under nitrogen.

Catalyst D (Comparative catalyst)

Catalyst D was prepared from catalyst A: 50 g of catalyst A were heatedat 300° C. in a quartz tube at atmospheric pressure in a stream of purenitrogen (35 l/h) for 30 hours and subsequently cooled to roomtemperature under nitrogen.

Catalyst E

Catalyst E was prepared by adding 6300 g of titanium dioxide to asolution of 1275 g of tungstic acid in 7300 g of 25% strength aqueousNH₃ solution. This mixture was kneaded for 60 minutes and then dried for12 hours at 120° C. The powder obtained after sieving was tableted andthe resulting pellets (3×3 mm) were subsequently calcined for 2 hours at690° C. The catalyst had a tungsten content, calculated as tungstentrioxide, of 20% by weight, based on the total weight of the catalyst

Catalyst F

Catalyst F was prepared from catalyst E: 50 g of catalyst E were heatedat 300° C. in a quartz tube at atmospheric pressure in a stream of purehydrogen (35 l/h) for 12 hours and subsequently cooled to roomtemperature under argon.

Catalyst G

Catalyst G was prepared by adding 200 g of titanium dioxide to asolution of 53 g of tungstic acid in 200 g of 25% strength aqueous NH₃solution. This mixture was kneaded for 150 minutes and then dried for 12hours at 120° C. The powder obtained after sieving was extruded and theresulting extrudates (2.5 mm) were subsequently calcined for 2 hours at675° C. The catalyst had a tungsten content, calculated as tungstentrioxide, of 20% by weight, based on the total weight of the catalyst.

Catalyst H

Catalyst H was prepared from catalyst G: 100 g of catalyst G were heatedat 300° C. in a quartz tube at atmospheric pressure in a stream of purenitrogen (40 l/h) for 30 minutes, 4 l/h of hydrogen were subsequentlymixed into the stream of nitrogen and the catalyst was reduced for 12hours at 300° C. using this gas mixture. It was subsequently cooled toroom temperature under nitrogen.

Catalyst I

Catalyst I was prepared by adding 18 kg of zirconium hydroxide to asolution of 4.25 kg of tungstic acid in 23.1 kg of 25% strength NH₃solution. This mixture was kneaded for 30 minutes and then dried for 11hours at 120° C. The powder obtained after sieving was tableted and theresulting pellets (3×3 mm) were subsequently calcined for 4 hours at675° C. The catalyst had a tungsten content, calculated as tungstentrioxide, of 20% by weight, based on the total weight of the catalyst.

Catalyst K

Catalyst K was prepared from catalyst I: 50 g of catalyst I were heatedat 300° C. in a quartz tube at atmospheric pressure in a stream of purehydrogen (35 l/h) for 29 hours and subsequently cooled to roomtemperature under argon.

Batchwise THF polymerization

The batchwise polymerization experiments were carried out at atmosphericpressure under a nitrogen atmosphere in 100 ml glass flasks fitted withreflux condensers. 10 g of shaped catalyst bodies which had been driedfor 18 hours at 180° C./0.3 mbar before use to remove adsorbed waterwere heated at 50° C. in 20 g of butanediol-containing THF (butanediolconcentration: 2000 ppm) for 24 hours. Water-containing THF (1% of H₂ O)was subsequently added to the reaction mixture and the catalyst wasseparated off by filtration. After washing the catalyst three times with20 g of THF each time, the filtrates were combined and evaporated at 70°C./20 mbar on a rotary evaporator and subsequently for 30 minutes at150° C./0.3 mbar in a bulb tube. PTHF obtained as distillation residuewas weighed and analyzed by means of gel permeation chromatography(GPC). Table 1 shows the test results obtained from the catalysts A toK.

The polydispersity D as a measure of the molecular weight distributionof the polymers prepared in the examples was calculated from the ratioof the weight average molecular weight (M_(w)) and the number averagemolecular weight (M_(n)) according to the equation

    D=M.sub.w/ M.sub.n.

M_(w) and M_(n) were determined by means of GPC, with a standardizedpolystyrene being used for calibration. From the chromatogram obtained,the number average M_(n) was calculated according to the equation

    M.sub.n =Σc.sub.i /Σ(c.sub.i/M.sub.i)

and the weight average M_(w) was calculated according to the equation

    M.sub.w =(Σ(c.sub.i *M.sub.i))/Σci,

where c_(i) is the concentration of the individual polymer species i inthe polymer mixture and M_(i) is the molecular weight of the individualpolymer species i.

                  TABLE 1                                                         ______________________________________                                                            Catalyst pre-                                                                           Yield                                                                              M.sub.n                                                                             D                                      Example Catalyst treatment [%] (GPC) (GPC)                                  ______________________________________                                        1    (Compari- A        --      29   6680  2.7                                   son)                                                                         2  B H.sub.2 44 9710 2.8                                                      3  C H.sub.2 38 7260 2.6                                                      4 (Compari- D N.sub.2 30 7680 2.6                                              son)                                                                         5 (Compari- E -- 18 5580 2.5                                                   son)                                                                         6  F H.sub.2 21 3970 2.4                                                      7 (Compari- G -- 36 6850 3.9                                                   son)                                                                         8  H H.sub.2 39 6870 3.7                                                      9 (Compari- I -- 30 9090 3.6                                                   son)                                                                         10   K H.sub.2 33 9300 3.7                                                  ______________________________________                                    

In all cases, the catalysts activated by reduction display a significantyield increase compared with the unreduced catalysts and compared withthe catalyst from Comparative Experiment 4 in which hydrogen wasreplaced by nitrogen. This Comparative Experiment 4 shows that theeffect of the activation is achieved not only by a further thermaltreatment of the catalyst, but by treatment with a reducing agent.

The number average molecular weight of the PTHF obtained remainsessentially unaffected. Fluctuations of this value within the observedranges can be easily compensated, where desired, by adapting the telogencontent. The polydispersity, ie. the width of the molecular weightdistribution obtained, which is an important measure of the quality ofthe polymerization process and the properties of the polymer obtained,is not significantly altered by the activation of the catalyst.

The examples show that, by virtue of their significantly increasedactivity, the catalysts of the present invention lead to considerablyimproved economics of the process.

What is claimed is:
 1. A process for preparing polytetrahydrofuran,copolymers of tetrahydrofuran and 2-butyne-1,4-diol, diesters of thesepolymers with C₂ -C₂₀ -monocarboxylic acids or monoesters of thesepolymers with C₁ -C₁₀ -monocarboxylic acids, which comprisespolymerizing tetrahydrofuran in the presence of at least one of thetelogens water, 1,4-butanediol, 2-butyne-1,4-diol, polytetrahydrofuranhaving a molecular weight of from 200 to 700 Dalton, a C₁ -C₂₀-monocarboxylic acid or an anhydride of a C₂ -C₂₀ -monocarboxylic acidor a mixture thereof over a heterogeneous supported catalyst, whichcatalyst comprises a catalytically active amount of an oxygen-containingmolybdenum or tungsten compound or both on an oxidic support materialand which has been calcined at from 500° C. to 1000° C. afterapplication of the precursor compounds of the oxygen-containingmolybdenum and/or tungsten compounds onto the support materialprecursor, wherein the catalyst is activated by treatment with areducing agent prior to use as a polymerization catalyst.
 2. The processof claim 1, wherein the reducing agent used is a hydrogen-containinggas.
 3. The process of claim 2, wherein the reducing agent used is amixture of hydrogen and an inert gas.
 4. The process of claim 3, whereinthe inert gas used is argon or nitrogen or a mixture thereof.
 5. Theprocess of claim 4, wherein the proportion of hydrogen in the gasmixture is changed over the course of the reduction.
 6. The process ofclaim 1, wherein the reduction of the catalyst is carried out at fromabout 20 to 500° C.
 7. The process of claim 6, wherein the reduction ofthe catalyst is effected by a hydrogen-containing gas at a temperatureof from about 100 to 400° C.
 8. The process of claim 6, wherein thereduction of the catalyst is effected by a solid, liquid or dissolvedreducing agent at a temperature of from about 20 to 200° C.